It is known in the prior art to provide a driver's side inflatable air bag and place it within the steering wheel underneath an impact plate. It is also known from the prior art to provide an inflatable air bag for the other passenger seats, and in particular, the front passenger seat, whereby the passenger side air bag is inflated by ignition of a propellant charge, which is triggered by a signal detected by a known acceleration sensor whenever there is a heavy impact experienced.
A disadvantage which arises with current state of the art passenger side air bags is the triggering of an air bag when the passenger seat is not occupied. A needlessly inflated air bag creates a sudden pressure wave within the passenger compartment, which may be harmful to the driver and other passengers. Furthermore, when a passenger side air bag is inflated in the absence of a passenger to be protected, considerable and unnecessary repair or replacement costs result in order to restore the system. This is especially true if the passenger side air bag has been integrated with other vehicle parts, (e.g., as the dashboard) and the parts are torn off or broken at pre-determined breaking points. Thus, it becomes very costly to repair the car interior after the inflatable air bag has been used. In the event of a minor accident with a modest repair cost, a needless triggering of a passenger side air bag may make the cost of repair so high that it may be more economical to consider the vehicle as totalled.
The prior art is replete with methods to control the triggering of a passenger side air bag so that it is inflated only when the passenger may benefit from its use. It is known from DE-OS 21 34 590 and DE-OS 37 02 825 how to arrange a seat contact switch exposed on the passenger seat so that the switch responds mechanically to the weight of a seated passenger in order to detect the presence or absence of a passenger on that particular seat. Each seat contact switch is connected to a control unit which controls the triggering of each air bag such that the air bag for a particular passenger is only triggered when that passenger has been detected on the seat by the appropriate seat contact switch.
A safety device having a seat contact switch of the above type is also known from DE-OS 25 16 185. This known safety device discloses a seat contact switch which is used to detect an empty seat, whereby the associated air bag is still triggered, but is only partially inflated, thus posing a lesser risk of sudden-pressure and related safety problems associated with the needless inflation of an unused air bag.
From the above described prior art safety systems, it becomes evident that two operating conditions must be simultaneously satisfied for the useful activation of the passenger side air bag. First, there must exist an appropriate level of severity of an accident such that the acceleration sensor will send a signal which has exceeded a pre-set threshold value to the control unit which controls the triggering of the passenger side air bag. And second, a passenger must physically occupy the particular passenger seat in order to activate the seat contact switch and send the signal corresponding to "seat occupied condition" to the control unit so that it knows that all conditions are satisfied to inflate the air bag.
A common disadvantage with the above described methods exists since, in many instances, the seat contact switch does not respond to the presence of a passenger when that passenger is standing (such is often the case with children), kneeling on the floor, or sitting in a forward portion of his seat. When the seat switch does not respond, the air bag is not triggered by an impact, thus rendering the protective system ineffective.
DE-PS 38 05 887 teaches to provide a seat contact switch which is mechanically activated by the body weight of a sitting person whereby the seat switch may detect the body weight of a person as he shifts his weight over a broad area of the seat surface. The seat contact switch comprises a sandwich-type switch mat, having a pair of spaced contact layers which are separated by an elastically deformable, electrically insulating intermediate layer. This intermediate layer, being electrically conductive in the vertical direction, completes the circuit between the two contact layers as it is elastically deformed (compressed) when under the influence of the body weight of a seated person.
Seat switches of non-contact operation are also known in the prior art and are disclosed in DE-OS 36 35 644 wherein the seat switches comprise a condenser having two electrodes opposite to each other on the seat, such that an electrical field is set up between them. The capacity of the condenser changes with respect the presence or absence of a person on the seat. By sensing this capacity change, it is possible to detect the presence or absence of a person on the seat
It is also known from DE-OS 38 09 074 how to determine various sitting positions or the position of the center of gravity of the seated passenger, such as in a forward bending position, by placing several known pressure sensors in a pre-selected arrangement on the seat. This German patent document also teaches how to activate and inflate an associated air bag in accordance with the particular sitting position of the seated passenger.
In another known device for recognizing whether a vehicle seat is occupied or not, a distance measuring sensor is used instead of a seat switch to detect the presence of a passenger. This type of distance measuring sensor is typically provided on the dashboard and uses a radar-type method wherein a pulse is emitted in a direction towards the seated passenger and/or seat back. The pulse and echo are then reflected back either by the seat back (if the seat is empty) or by the stomach or chest region of the passenger (if the seat is occupied). The distance between the dashboard and the seat back or passenger is calculated from the pulse-echo travel time process and thus, the presence or absence of a person in the vehicle seat can be determined. This information is then used to determine whether or not the air bag for the passenger seat should be activated. This method requires the use of additional sensors to determine the particular position of a seat to account for horizontal fore-aft adjustment and angular tilt of the seat back. While this type of pulse-echo travel time process makes it possible to determine whether the region in front of the seat is occupied by a standing, kneeling, or sitting person, (while the traditional seat sensor would indicate the seat as empty), it is very costly to implement in view of the large manufacturing and installation expenses associated with the many seat location sensors involved.
Thus, there is a significant need in the art to provide a simple and low cost safety device for passenger side inflatable air bags to detect a passenger irrespective of whether the passenger is seated or positioned in front of the passenger seat. There is also a need in the art for an improved safety detection device which is able to distinguish between the presence of a passenger from luggage or other objects placed in front of or on the passenger seat, so that the inadvertent activation of the passenger side air bag is not caused to occur.